The TeX-Live project on github (https://github.com/TeX-Live) is a mirror of the upstream Subversion repository (https://tug.org/texlive/svn). We use it primarily for automatic building of the sources on several platforms.
We prefer using mailing lists to github: tex-live@tug.org for general bugs and discussion, tex-k@tug.org for bugs and patches for the compiled programs, and so on. List of TL lists: https://tug.org/texlive/lists.html.
However, if you prefer to open issues or submit PRs on github, that's ok. We will see them and will reply as soon as we can.
One common problem: Package bug reports should go to the package maintainer, however they wish to receive reports, not to any general TeX Live list. TL redistributes what is uploaded to CTAN without changes.
For information about the source tree here, see the ./README file here, not this README.md.